This invention relates to metal interconnections on semiconductor chips and more particularly to a surface coating or treatment to prevent surface atoms of conductors from being moved downstream by an electron current, a phenomena known as electromigration, and/or from being moved by a stress gradient tending to relax stress known as stress induced migration.
Increased density and speed of microelectronic circuitry has caused a shift in the metallization system from Al(Cu) to Cu to reduce the resistance of the conductor. At the same time, the need for higher levels of current density has exposed the Al(Cu) lines to failure by electromigration, limiting the extent to which circuit designers can increase performance. The higher melting point of Cu is expected to improve the current carrying capability of the conductors, extending electromigration lifetime. However, from our test results, we have found that the electromigration lifetime is most dependent on the atomic behavior such as atomic transport at the Cu/dielectric interface and not on the intrinsic character of the Cu lattice, grain boundary or Cu/liner interface atom transport. Thus, the specifications for allowed current in Cu conductors proved not to be significantly superior to those for Al(Cu).
The basic process for fabricating Cu conductor patterns is by the damascene method where trenches are formed in a dielectric layer, a liner is formed in the trenches and then the trenches are filled with Cu and then chemical mechanical polished (CMP) which terminates in a clean top Cu surface coplanar with the dielectric layer. Cu/metal liner interfaces are formed on the other three surfaces of the Cu conductor. The electromigration lifetime is a function of the line thickness, width and cross section area. Data from measurements showed that only the top surface was contributing to mass transport for Cu conductors with bamboo-like and near bamboo grain structure and a combination of boundary and surface were contributing to mass transport for wider lines. The same relationship was found to be independent of the Cu deposition method, i.e., whether the deposition was by Physical Vapor Deposition (PVD), Chemical Vapor Deposition (CVD) or electroplating, illustrating the pervasiveness of the problem. In addition to electromigration lifetime failure, it was found that Cu conductor patterns are susceptible to atomic voiding under thermal cycle stress, with voids always found to be initiated at the Cu/dielectric interface where the atomic mobility is highest. Since the copper migration rate, drift velocity, is a product of mobility (D/kT) times driving force (F) where D is the diffusivity of Cu, T is the absolute temperature in Kelvin, k is the Boltzmann constant. The driving forces, F, are Z*eE and (Ds/Dx) W for electromigration and stress induced migration forces, respectively, where Z* is the effective charge number, e is the electronic charge, E is the electric field, (Ds/Dx) is the stress gradient, and W is the atomic volume. It is now inevitable that achievement of a Cu conductor technology with which the highest performance and reliability can be realized will require alteration or modification of the Cu/dielectric interface of an overlayer to reduce Cu transport and atomic voiding.
Modification of the Cu conductor surface can be done after CMP. Then, the process should be selective to Cu and provide a thin layer on the surface of the Cu conductor so as to remain substantially coplanar with the dielectric or insulation containing the trenches. The process must result in improved adhesion between the Cu conductor and the subsequently formed dielectric over layer and improved oxidation resistance of the Cu atoms in the Cu conductor, as well as reduced atomic transport of the Cu conductor, to provide maximum reliability.
In accordance with the present invention, the exposed Cu surface of a Cu conductor is altered by providing a thin metal layer in the range from 1 to 20 nm on the Cu surface, such as subsequent to a CMP planarization or a dry etching process, in order to reduce susceptibility to electromigration, oxidation, corrosion, stress voiding and delamination during subsequent chip processing and/or chip utilization, thus improving reliability and yield. This invention teaches the method of electroless metal deposition as one way to form a thin metal layer on a Cu conductor to increase electromigration lifetime and stress induced migration resistance.
The selective deposition process comprises the following steps.
a. Selective activation of the copper surface using a Pd ion catalyst which is shown in Equation 1.
Cu+Pd++- - - xc3xa1Cu+++Pdxe2x80x83xe2x80x83(1)
This procedure leaves a multitude of Pd nanoparticles on the surface of the copper lines to act as a catalyst for the next step.
b. Selective deposition of a metal or alloy using electroless reduction which is shown in Equation 2.
reducing agent+Men++Cu - - - xc3xa1Cu/Me+oxidized form of reducing agentxe2x80x83xe2x80x83(2)
Where Men+ is a metal ion, for example, Co2+.
For example, using hypophosphite as the reducing agent and cobalt ions as the capping metal, we have a chemical reaction as shown in Equation 3.
2H2PO2xe2x88x92+Co2++H2O . . . ((. . . Cu . . . )) - - - xc3xa1Co+2HPO32xe2x88x92+H2+4H+xe2x80x83xe2x80x83(3)
In the course of this invention we have used the hypophosphite (capping layer) system described above, and deposited onto the surface of the copper conductor the metal palladium, cobalt, cobalt-tungsten, cobalt-tungsten-phosphide, cobalt-tin, cobalt-tin-phosphide, colbalt phosphide and other alloys of cobalt. It is also understood that other reducing agents can be used, such as dimethylamino borane, which allows deposition of other types of alloys, such as Coxe2x80x94B or Nixe2x80x94B.
As a result of steps a and b above, the exposed surface of the copper conductors are coated with a thin film preferably in the range from 1 to 10 nm thick of a metal compound or alloy which is strongly adhered via chemical and/or metalurgical bonds to copper atoms on or in the copper surface.
The following important findings are critical to this invention in a damascene process for making copper interconnects on integrated circuit chips.
1. The thickness of the coating film on the copper surface must be small: maximum 20 nm, and preferably between 1 and 10 nm due to damascene process considerations. In this way, at least three major properties are achieved: a) the resistivity of the Cu line is not affected or increased by more than 20%, b) the electrical leakage, which may occur due to bridging between Cu conductors, is eliminated and c) no further planarization of the Cu conductor or line is necessary. Planarization or substantial planarization of the Cu conductor/dielectric surface is required in order to provide a planarized top surface of a subsequently formed dielectric layer thereover without the need to planarized the top surface of the subsequent dielectric layer. Step features from one surface are translated through the next layer to the surface above especially when formed by CVD.
2. The preferred composition of the coating films may be Coxe2x80x94Snxe2x80x94P, Coxe2x80x94Wxe2x80x94P, Coxe2x80x94P or Pd.
3. The wafers with coated films were annealed at 300xc2x0 C. for two (2) hours to further improve the chemical or metalurgical bonding to the copper surface and slow down diffusion of the copper atoms along the Cu surface at the Cu conductor/dielectric interface and along grain boundaries. The Cu conductor/dielectric interface is between the Cu conductor and the dielectric layer formed over the Cu conductor. The Cu conductor in a damascene process is in a trench in a dielectric layer with a metal liner formed in the trench prior to filling with the Cu conductor.
4. Also, selective CVD of metals, or other deposition processes may be used to deposit a metal film on the surface of the Cu conductor with chemical and/or metalurgical bonding to achieve increased electromigration resistance of the Cu conductor.